The present invention relates generally to the field of rotary die cutting devices which are useful for manufacture of carton blanks and other workpieces from sheets or webs of material, and particularly to rotary die cutting machines used in high speed rotary cutting of a moving sheet or web of paper, paperboard, plastics and composite materials.
As one example, rotary die cutting machines are used for the high speed mass production of paperboard carton blanks that are subsequently folded into the shape of carton or box containers. In rotary die cutting, the cutting operations can occur by either of two methods, the first being referred to in the trade as rotary pressure (xe2x80x9cRPxe2x80x9d) cutting and the second as the xe2x80x9ccrush cutxe2x80x9d method. In the rotary pressure cutting method, the paperboard material from which the carton blanks are generated is advanced at high speed between two rotary die cutting cylinders. The cylinders are provided with cutting elements on them which cooperate to form cutting dies to cut the desired shape of the carton blank as the material advances between the cylinders. The cylinders rotate at the same speed so as to maintain registration of the co-acting cutting surfaces. Each rotation of the cylinders generates a discrete series of one or more cartons depending upon the size of the cartons, diameters of the cylinders, etc. In the crush cut method, one of the cylinders, the cutting cylinder, is provided with knife blades that perform the cutting operation and the other cylinder, known as the anvil cylinder, provides a smooth surface against which the knife blades operate.
In both the above described processes there is necessarily generated a certain amount of scrap material. This material needs to be separated from the carton blanks and removed from the dies as each revolution of the cylinders generates a new series of carton blanks. There are various ways in which the scrap removal process is conventionally performed. In one method, the scrap material is initially retained on one of the die cylinders by stripping pins that hold the scrap pieces onto that cylinder as the carton blanks are advanced away. Thereafter, the scrap pieces are removed from the die cylinder by a stripping comb. Alternatively, scrap removal is accomplished separately from the cutting operation. In this method, the carton blank and scrap pieces are retained as contiguous pieces by leaving uncut during the cutting operation. The attached pieces are then carried to a stripping station. At the stripping station, the scrap material is removed from the carton blank by piercing the scrap portions with stripping pins carried on a rotating cylinder. As in the previous method, the scrap pieces are retained on the pins by the rotating cylinder until they are stripped off the pins by a stripping comb. A yet further alternative system employs a stripping station which removes the scrap pieces by rotatably registering male elements to xe2x80x9cpunchxe2x80x9d the scrap from the moving web of pre-cut products.
All of the above described methods of scrap removal are expensive operations that add significantly to the cost of rotary die cutting and require time and labor intensive adjustments to optimize their operation in a coordinated fashion with the die cutting operation. For example, the location and number of stripping pins varies for each die and their installation can involve a certain amount of trial and error. Further, each of the stripping pins must be individually mounted to the die. For this purpose, each stripping pin is typically provided with a screw threaded base which mounts within complementary threaded mounting holes tapped into the portions of the die where the scrap material is generated. In addition, for each stripping pin a corresponding registration hole must be drilled or otherwise formed in the opposing die. These holes must be precisely located to register with the stripping pins, and so the need for these holes also increases the cost of the dies. A further disadvantage arises due to the size of the stripping pins. Occasionally, the scrap pieces are very small and it is difficult due to the minimum practical size of the pins and mounting holes to locate them where they are needed. Since the pin locations vary for each die, the stripping combs must also be custom-built for each die, it being necessary for the pins to pass between the teeth of the comb as the scrap material is being stripped from the die.
An improved rotary die cutting apparatus and method for removing scrap material from work pieces such as paperboard blanks. One of the dies of a pair of rotary cutting dies is provided with gripper elements that extend over at least a portion of the area of the die corresponding to the areas over which the scrap portions are generated. The gripper elements grip, without piercing completely through, the scrap portions generated by the cutting operation of the dies. A stripping knife extends across the die carrying the scrap portions at a height above the gripper elements that provides clearance between the gripper elements and the stripping knife but contacts the scrap portions retained on the gripper elements. The stripping knife dispenses with the need for multiple custom built stripping combs corresponding to each set of dies. The gripping elements dispense with the need for individually mounted stripping pins and corresponding registering holes. In one preferred aspect of the invention, the gripper elements are formed integrally with the rotary cutting die by employing conventionally known masking and etching techniques. The invention can be employed with rotary cutting dies employing either rotary pressure cutting or crush cut methods of operation.